Hi Jim,
I am also using silk to cover open bays. The last I got was from Darhma Trading or Thai Silks. The cost is very reasonable! Much less expensive than "Model Aircraft" silk, maybe less than 1/3rd the cost. Haboti silk is the type and the weights are either 5 momme or 8 momme. I really like using it ad it does give more durability thank silkspan but maybe not quite as much as Polyspan.
For the dope, use tautening dope only to shrink the covering between open bays, once it is shrunk like you like it, switch to non tautening. Also use non tautening to build the substrate, stick down the covering and make your filler from.
BIG Bear
RNMM/AMM